1. Field of the Invention
The present invention relates to a wide-angle lens and more particularly to a retrofocus type wide-angle lens with a large aperture in which a rear lens group is moved in the focusing operation.
2. Related Background Art
There are retrofocus types of wide-angle lenses. In general, a focusing system for retrofocus type wide-angle lenses, includes a rear focus type or an inner focus type in which a few lenses are moved in the focusing operation to improve close distance focusing performance and operation. Various retrofocus wide-angle lenses have been proposed, e.g., U.S. Pat. Nos. 4,029,397 and 4,806,003 and Japanese Patent Application Laid-Open Nos. 4-191715 and 1-134411.
In the retrofocus type wide-angle lens disclosed in U.S. Pat. No. 4,029,397, a first lens group has a positive lens and a negative lens in that order from the object side. A second lens group basically has a power (refracting power) arrangement of the retrofocus type wide-angle lens. A master section (a rear lens group having a positive refracting power) of the second lens group is in the form of a modified Triplet. As the focusing system, a rear focusing system is adopted in which the second lens group is moved in the focusing operation while the first lens group is fixed.
In this retrofocus type wide-angle lens with the rear focusing system, the first lens group has the positive lens and the negative lens arranged in that order from the object side, i.e., takes the positive lens preceding form. Therefore, principal rays of marginal pencils of rays pass at positions far from the optical axis in an object-side lens surface of the retrofocus type wide-angle lens located closest to the object side. As a result, the diameters of the front lenses are enlarged and the lateral chromatic aberration as well as the distortion in the peripheral area are increased, which is disadvantageous in terms of performance and miniaturization.
Further, because the structure of the master lens section (the rear lens group having the positive refracting power) of the second lens group is a modified Triplet type (or a tele-sonnar type) the same as in general retrofocus type wide-angle lenses, it is difficult to make the aperture large.
In the retrofocus type wide-angle lens disclosed in Japanese Patent Application Laid-Open No. 4-191715, although a first lens group has a negative lens and a positive lens arranged in that order from the object side, i.e., takes the negative lens preceding form, the first lens group has a positive refracting power as a whole. Therefore, it is difficult to make an angle of view of wide-angle lens larger, and the size of the wide-angle lens is likely to be enlarged. Also, the structure of the master section (the rear lens group having a positive refracting power) of a second lens group is a modified Triplet type, the same as in the above gazette, so it is difficult to make the aperture large.
Further, in the retrofocus type wide-angle lens disclosed in U.S. Pat. No. 4,806,003, the lens system consists of three lens groups having negative, positive and positive refracting powers in that order from the object side, and the rear two lens groups are moved by respectively different amounts in the focusing operation. Basically, the first lens group with the negative refracting power constitutes a wide converter and the second and third lens groups with the positive refracting powers constitute a Gauss type high speed lens.
Thus, the second and third lens groups constitute the Gauss type high speed lens, whose structure is advantageous to make the aperture large. However, in order to keep a sufficient back focus and to obtain a wider angle as a retrofocus wide-angle lens by this structure, there is no other way but for the refracting power of the first lens group to be made considerably large. In such a case, imaging performance is deteriorated and the size of the wide-angle lens is enlarged.
Furthermore, in the retrofocus type wide-angle lens disclosed in Japanese Patent Application Laid-Open No. 1-134411, the lens system consists of three lens groups having negative, positive and positive refracting powers in that order from the object side and the second lens group with the positive refracting power is moved in the focusing operation. Basically, the first lens group with the negative refracting power constitutes a wide converter and the second lens group with the positive refracting power constitutes a retrofocus type wide-angle lens. And, the third lens group constitutes a modified Gauss type lens.
Accordingly, the above structure is effective to obtain a large aperture and a large angle of view. However, despite the fact that the above structured wide-angle lens is big in size and has a large number of lenses, aberrations are not corrected sufficiently. In particular, the corrections of the coma and the lateral chromatic aberration are not performed sufficiently. Also, in manufacture, it is difficult to highly stabilize the sensitivity of the decentration of each lens element.